Plastics compositions find a wide range of industrial applications because of good strength to mass ratios combined with ease of processability and ready availability. Depending upon the properties sought in a plastics article, a wide range of polymeric species is available with vastly differing physical properties. The properties of a polymeric species, in turn, can be modified by the addition to or the incorporation in the polymeric species of modifiers such as fillers and reinforcing agents, antioxidants, mould release agents, UV stabilizers, pigments, lubricants, plasticizers, impact modifiers, flame retardants and the like.
In the formulation of plastics compounds there are usually finite and practical limitations to which the physical properties of a particular polymeric species can be modified before economic benefits and/or deterioration of other physical properties becomes a consideration. For example, it is known that the addition of a carbon black filler to rubber polymers initially contributes greatly to physical strength but beyond certain limits those strength properties are reduced. Similarly, the incorporation of fillers such as calcium carbonate, glass fibres or the like to thermoplastic resins becomes uneconomical after a relatively small increase in the specific gravity of the compounded plastics material.
Generally speaking, most plastics compositions are inherently good electrical insulators. In some cases however, it is desirable for plastics articles to exhibit a degree of electrical conductivity to dissipate electrostatic charges in electronic devices such as mobile telephones, portable computers and the like to avoid potentially damaging electrostatic discharges.
Finely divided carbon black is probably the best known additive to improve electrical conductivity in plastics compositions. While inexpensive, a relatively large quantity of carbon black needs to be incorporated into polymers to achieve any appreciable level of conductivity. In many cases, the quantity of carbon black required to achieve a useful degree of conductivity is sufficient to substantially degrade physical properties such as impact and tensile strength and Young's modulus. Moreover, as the “wettability” of carbon black by some polymers, particularly polyolefins, is poor, this leads to poor dispersion which can give rise to surface blemishes delamination, surface degradation and otherwise reduced physical properties.
Of more recent times, it has been proposed to utilize carbon nano-particles as electroconductive fillers. These nano-particles typically have sub-micron dimensions and are selected from vapour grown carbon fibres (VGCF) or carbon nano-tubes (CNT). While very effective as conductive fillers, these carbon nano-particles are much more expensive than carbon black fillers. For example, VGCF fillers are about twenty times the cost of carbon black fillers, whereas CNT fillers are from 10-1000 times the cost of VGCF fillers.
J Sandler et al (Polymer, 40, (1999) 5967) indicated that in order to avoid electrostatic charging of an insulating matrix, an electrical conductivity of σ=10−6 Ωm (or a resistivity of ρ=106 Ωm) was required. By incorporating carbon nano-tubes into an epoxy resin matrix with intense stirring, a matrix conductivity of σ=10−2 Ωm was reported with filler volume fractions as low as 0.1 vol %.
Jun Xu et al (Composites: Part A, 35 (2004) 693) reported that vinyl ester resin based composites with 8% VGCF content showed a resistivity of p=102 Ωm and a percolation threshold pc between 2-8 wt % of VGCF.
In other journal articles, K. Logano et al (Journal of Applied Polymer Science, 80 (2001) 1162) reported that 15-20 wt % was required to be incorporated into a polypropylene (PP) matrix to achieve a volume resistivity of ρ=106 Ωm, while S. A. Gordeyev et al (Physica B, 279 (2000) 33) reported that the percolation threshold for PP/VGCF composites was about 4-5 vol %.
Japanese Publication Number JP 2004-300244A described epoxy-based composite thin sheets with a volume resistivity of ρ=100 Ωm or less with a VGCF content of 13 wt % in the presence of a carboxy terminated butadiene acrylonitrile (CTBN) rubber phase. This is a solvent based system suitable for thin film applications.
U.S. Pat. No. 6,528,572 discloses a conductive composition comprising a polymeric resin, an electrically conductive filler selected from carbon fibres, VGCF and CTN fillers, carbon black, conductive metal fillers, conductive non-metal fillers and the like or mixtures thereof and an incompatible antistatic agent in the form of a block copolymer.
U.S. Pat. No. 5,213,736 describes a process for making an electroconductive polymer composition having a matrix comprising a mixture of incompatible polymers wherein VGCF is preferentially distributed predominantly throughout one of the polymers which exhibited a higher affinity for VGCF.
United States Patent Application Publication No US 2003/0181568 A1 discloses a conductive plastics composition comprising a thermoplastics resin to which is added either carbon powder (up to 25 wt %) or glass fibres (up to 50 wt %) or a combination of both and VGCF filler (up to 30 wt %). At 15 wt % of glass fibre filler, surface resistivities of from 105 to 109Ω were obtained while at 20 wt % of glass fibre filler, surface resistivities in the range of from 107 to 1012Ω were reported.
While the prior art conductive polymeric compositions may be generally satisfactory for their respective intended uses, many are quite polymer specific and are limited to the extent that other desirable properties are not readily conferred without compromising the electrical conductivity properties.
Similarly, the processability of many of these prior art electrically conductive polymeric compositions is limited only to thin film solvent based applications or only to thermoplastic moulding techniques.
It is an aim of the present invention to overcome or ameliorate at least some of the difficulties associated with prior art electroconductive polymer compositions and/or at least provide consumers with a more convenient choice.